Set a cooling or baking rack into a baking sheet. Place the chicken pieces on the rack and drizzle with 2 tablespoons olive oil. Rub the spice mixture all over. Rub some of the mixture in between the skin and the breast meat. Transfer to the oven and roast until a thermometer inserted into the thickest part of the breast registers 160 degrees F and the juices run clear when pierced with a fork, 50 minutes to 1 hour. Remove the chicken from the oven and cover with some aluminum foil. Let rest for 15 minutes.

Meanwhile, preheat the remaining 2 tablespoons olive oil in a heavy medium saucepan over medium-high heat. Add the onions and saute until translucent, about 5 minutes. Add the garlic and cook until fragrant, about 2 minutes. Add the tomato sauce and chicken broth and cook over medium-high heat until the flavors incorporate, about 8 minutes. Season with salt and pepper. Cool slightly. Transfer the mixture to a blender with the soaked pasilla chiles and puree until smooth. Return to the same saucepan and add the chopped chocolate, stirring over medium heat until fully melted, about 3 minutes. Season with salt and black pepper. Transfer the chicken to a serving platter. Top with the chocolate pasilla sauce. Sprinkle with toasted sesame seeds and serve.